In a liquid crystal display and other flat panel displays, a phase plate, a polarizing plate and a liquid crystal cell substrate, etc. configured by including a thermoplastic resin film are used. As the phase plate, those obtained by stretching a raw material film, such as polycarbonate, giving a certain amount of retardation (also referred to as a phase difference) thereto, and putting two or more of them together are mainly used.
As the polarizing plate, those obtained by stacking a protective film on an upper surface and a lower surface of a polarizer made by polyvinyl alcohol are mainly used. As the protective film of a polarizer, a film obtained by a solution casting method of triacetyl cellulose is mainly used. As the liquid crystal substrate film, a film made by polyethylene terephthalate (PET) is mainly used.
Along with a recent flat panel display becoming larger, thinner and finer and to have an increased contrast, the above various thermoplastic resin films have been required to have highly less optical distortion than those in the conventional ones. It is because when the optical distortion is large, optical defects, such as color unevenness and color absence, etc., allover the flat panel display are caused.
Conventionally, as an index for indicating optical distortion of various thermoplastic resin films, an amount of retardation has been mainly used. By making the value of the retardation amount small or a predetermined value and uniform on a plane, an effort to diminish the optical distortion has been made. For example, in a polarizer protective film, having a small and constant retardation amount allover the film has been its index.
An optical film made by a thermoplastic resin wherein a retardation amount is small and constant allover the film has been proposed.
As a polarizer protective film, conventionally, a film made by a solution casting method of triacetyl cellulose (TAC) has been mainly used. A solution casting method film of TAC has a relatively small retardation being relatively constant on a film surface. However, there are disadvantages that it is poor in productivity, a solvent cannot be completely removed at the time of drying after the solution casting, and an emitted solvent gives an adverse effect on an electronic circuit and other parts of the flat panel display to cause an erroneous operation and display defective, etc.
Therefore, recently, as a protective film, a film obtained by a melt extrusion method of a thermoplastic resin has been studied as substitute for the film by a solution casting method of TAC. For example, the Japanese Unexamined Patent Publication No. 2000-273204 discloses a technique of obtaining a sheet having a sheet thickness of 150 to 1000 μm, in-plane thickness tolerance (Rmax) on the sheet of 15 μm, roughness of the sheet surface of 0.06 μm, and a retardation amount (maximum value) on the sheet surface of 15 nm, wherein a retardation amount is relatively constant and relatively small by performing melt extrusion molding on a specific thermoplastic resin under a specific condition.
However, the technique disclosed in the publication is for a thick sheet of 150 μm or more, generally, the thinner the sheet thickness becomes, the larger the thickness unevenness tends to be comparing with the thickness, and optical distortion becomes worse. Therefore, there has been a problem that it was impossible to respond to a recent demand for a thinner flat display.
Also, in the technique disclosed in the publication, there was a problem that a maximum value of an in-plane retardation amount was as much as 15 nm, which caused large optical distortion, so that it was impossible to respond to required performance of a flat panel display with still higher performance.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.